EDIT 23/1/2012- I've edited this topics subject as it seems that its not the lens but the body that has the problem. I'll not edit the various posts so folks can see the whole story.

It was pointed out by Gregory.Rotter HERE that this camera/lens combination was front focusing. Now because I'm a newbie with an untrained eye I hadn't noticed this > after a bit of google action I came across this test sheet, downloaded it and ran the test

There is no such thing that this lens and that camera = front or back focus.
This is entirely based upon chance and it is an example of completely normal production variation, to get this adjusted properly as far as I understand you need to give the company both your lens and your camera and calibrate them together.

Because of this problem I refuse to buy a camera without the "Micro Adjust" feature.

Not sure how it will work with Canon and a 3rd party lens? Your camera works great with the 50mm. Can Sigma take the camera and lens then adjust the lens to match the camera? If you go to Canon and have the camera adjusted then you 50mm will probably be off.